Tornadoes form from large thunderstorms called
super cells. Meteorologists are not quite sure
what causes some storms to produce tornadoes while
others do not. What is known is that there is
circulation inside of a thunderstorm. Warm air
rises up and cools with altitude until it becomes
unstable and descends again. This rotation
generally happens parallel to the ground and
produces nothing more than rain, hail, and
lightning.

If the storm twists and the rotation
becomes perpendicular to the ground, a tornado can
form. Officially, to be labeled a tornado the
rotating column must be in contact with the ground
and the storm cloud.